A related patent application is a commonly assigned Japanese Patent Application No. 2001-240738 filed on Aug. 8, 2001, which is incorporated by reference into the present patent application.
1. Field of the Invention
The present invention relates to a power module, and more particularly to a power module having a plurality of insulated gate bipolar transistors (IGBTs) arranged in parallel with each other.
2. Description of the Related Art
FIG. 4 is a schematic view of a conventional power module indicated in its entirety as 500. The power module 500 includes a ceramic substrate 501. On the ceramic substrate 501, an emitter electrode pattern 502, a collector electrode pattern 503, and a gate electrode pattern 504 are provided in substantially parallel with each other. At each one end of the electrode patterns 502, 503, and 504, an emitter lead 512, a collector lead 513, and a gate lead 514 are provided, respectively.
Onto the collector electrode pattern 503, two insulated gate bipolar transistors (IGBTs) 521 and 522 are fixed. Each back face of the IGBTs 521 and 522 is a collector electrode. Each of the collector electrodes is electrically connected to the collector electrode pattern 503. Emitter electrodes on the front faces of the IGBTs 521 and 522 are connected to the emitter electrode pattern 502, via bonding wires 523 and 524, respectively. Gate electrodes on the IGBTs 521 and 522 are connected to the gate electrode pattern 504 via bonding wires 525 and 526, respectively. Furthermore, an emitter electrode on the IGBT 521 is connected to a ground electrode 528, via a bonding wire 527. Typically, a cover is placed over the ceramic substrate 501 to seal the IGBT 521 and other components. However, the cover is omitted in this drawing.
FIG. 5 shows a layout of the emitter electrode pattern 502 and the collector electrode pattern 503 of the power module of FIG. 4. In FIG. 5, like reference numerals refer to similar or corresponding parts shown in FIG. 4.
In such power module 500, the two IGBTs 521 and 522 are arranged in parallel with each other. Feeding signals into the gate electrode pattern 504 allows simultaneous switching of the two IGBTs 521 and 522, thereby supplying large current between the emitter electrode pattern 502 and the collector electrode pattern 503.
However, when the IGBTs 521 and 522 are switched, the inductance components generate counter electromotive force in the bonding wires 525 and 526, and the emitter electrode pattern 502.
First, suppose that electric potential at each point of P, Q, R, and S shown in FIG. 5 is 0 V (ground potential) in the initial state.
Next, the IGBTs 521 and 522 are switched using the bonding wires 525 and 526, respectively, and brought to the ON state. Immediately after switching, current flows from the point P to the point Q, from the point Q to the point R, and from the point S to the point R. Then, current flows through the emitter electrode pattern 502 in the direction shown by an arrow 530. At the same time, the inductance components cause voltage drop between the points P and Q, and the other points. As a result, unbalanced conditions occur in the gate-to-emitter voltages on the IGBTs 521 and 522. These unbalanced conditions cause a timing lag of switching the IGBTs 521 and 522.
For example, suppose that the electric potential at the point P is 0 V. Then, the electric potential at the point Q is xe2x88x923 V and that at the point R is xe2x88x925 V because of the influence of the voltage drop. On the other hand, voltage increases in the direction from the point R to the point S, and thus the electric potential at the point S is xe2x88x922 V. Therefore, the gate-to-emitter voltages on the IGBTs 521 and 522 have different values of 15 V and 17 V respectively, immediately after the electric potential at the both gates are set to 15 V. This phenomenon causes a timing lag of switching the two IGBTs 521 and 522. Such a timing lag results in excessive load imposed on one of the IGBTs, which has been one of causes of failures and shorter lives of the IGBTs.
The object of the present invention is to provide a power module having a plurality of IGBTs arranged in parallel with each other and switched at the same timing.
The present invention provides a power module having a plurality of power devices arranged in parallel with each other and switched by gate signals with substantially equal electric potential. The power module includes: a collector electrode pattern, first and second power devices provided on the collector electrode pattern and each having a collector electrode connected to the collector electrode pattern; an emitter electrode pattern provided along the collector electrode pattern and having an emitter lead, and first and second connection means for connecting emitter electrodes on the first and second power devices and the emitter electrode pattern, respectively. The power module is characterized in that an inductance component of at least one of the first and second connection means is adjusted so that the inductance component between the emitter electrode on the first power device and the emitter lead is substantially equal to that between the emitter electrode on the second power device and the emitter lead.
Also, the present invention provides a power module further including: a third power device provided on the collector electrode pattern in symmetrical relation with the first power device so as to sandwich the second power device together, and having a collector electrode connected to the collector electrode pattern, and a third connection means for connecting an emitter electrode on the third power device and the emitter electrode pattern. The emitter lead is provided substantially in the center of the emitter electrode pattern so that the inductance component between the emitter electrode on the first power device and the emitter lead is substantially equal to that between the emitter electrode on the third power device and the emitter lead. The inductance component of the second connection means is adjusted so that the inductance component between the emitter electrode on the second power device and the emitter lead is substantially equal to that between the emitter electrode on the first power device and the emitter lead.